Stockholders' Equity of the Company (Details Textuals) (USD $)	0 Months Ended	12 Months Ended			1 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended		0 Months Ended	12 Months Ended		0 Months Ended	12 Months Ended								1 Months Ended
	Apr. 16, 2012	Dec. 31, 2012 Quarters	Dec. 31, 2011	Dec. 31, 2010	Aug. 31, 2012 Series H Cumulative Redeemable Preferred Stock [Member]	Dec. 31, 2012 Series H Cumulative Redeemable Preferred Stock [Member]	Dec. 31, 2011 Series H Cumulative Redeemable Preferred Stock [Member]	Mar. 31, 2012 Series G Cumulative Redeemable Preferred Stock [Member]	Dec. 31, 2012 Series G Cumulative Redeemable Preferred Stock [Member]	Dec. 31, 2011 Series G Cumulative Redeemable Preferred Stock [Member]	Apr. 16, 2012 Series E Cumulative Redeemable Preferred Stock [Member]	Dec. 31, 2012 Series E Cumulative Redeemable Preferred Stock [Member]	Dec. 31, 2011 Series E Cumulative Redeemable Preferred Stock [Member]	Apr. 16, 2012 Series F Cumulative Redeemable Preferred Stock [Member]	Dec. 31, 2012 Series F Cumulative Redeemable Preferred Stock [Member]	Dec. 31, 2011 Series F Cumulative Redeemable Preferred Stock [Member]	Dec. 31, 2012 Series E and Series F Cumulative Redeemable Preferred Stock [Member]	Dec. 31, 2010 Common Stock [Member]	Dec. 31, 2012 Issuance of Equity under ATM Program [Member]	Dec. 31, 2011 Issuance of Equity under ATM Program [Member]	Dec. 31, 2012 Issuance of Equity under ATM Program [Member] Common Stock [Member]	Dec. 31, 2011 Issuance of Equity under ATM Program [Member] Common Stock [Member]	Aug. 31, 2012 Issuance of Equity [Member]	Feb. 28, 2012 Issuance of Equity [Member]	Apr. 30, 2011 Issuance of Equity [Member]	Apr. 30, 2010 Issuance of Equity [Member]	Aug. 31, 2012 Issuance of Equity [Member] Common Stock [Member]	Feb. 28, 2012 Issuance of Equity [Member] Common Stock [Member]	Apr. 30, 2011 Issuance of Equity [Member] Common Stock [Member]	Apr. 30, 2010 Issuance of Equity [Member] Common Stock [Member]
Class of Stock [Line Items]
Preferred stock, dividend rate					6.375%	6.375%		6.875%	6.875%		7.80%		7.80%	7.50%		7.50%
Preferred stock issued (in shares)					4,000,000	4,000,000		4,000,000	4,000,000				1,610,000			3,450,000
Liquidation preference (in dollars per share)					$ 25			$ 25	$ 25
Proceeds from issuance of preferred stock		$ 192,411,000			$ 96,200,000			$ 96,200,000
Number of quarters		6
Number of days within a change in control in which preferred stock may be redeemed									120 days
Redeemable preferred stock issued upon conversion, maximum						4,187,600			4,390,000
Preferred stock (in shares)						4,000,000			4,000,000		1,610,000		1,610,000	3,450,000		3,450,000
Redemption price (in dollars per share)											$ 25
Stock Redeemed or Called During Period, Value	126,500,000	126,500,000
Preferred stock redemption premium		6,980,000	0	0													4,900,000
Issuance of common stock, shares																		9,200,000			787,118	355,305					5,750,000	9,487,500	6,037,500	9,200,000
Proceeds from Issuance of Common Stock		672,102,000	233,312,000	299,847,000															36,300,000	12,800,000			253,800,000	382,100,000	221,000,000	299,800,000
At Market Stock Offering Program Aggregate Value Of Common Stock		200,000,000
At Market Stock Offering Program Gross Proceeds																			37,000,000	13,000,000
Common stock issued (in shares)		74,926,981	58,819,717																		1,142,423
At Market Stock Offering Program Remained Available For Issuance		$ 150,000,000
Dividend reinvestment plan, stock issued (in shares)			0
Share Repurchase, remaining number of shares authorized to be repurchased		988,025